A continuously variable transmission (CVT) capable of continuously changing the gear ratio is known. In the continuously variable transmission, for example, a metal belt or a chain is held by a primary pulley and a secondary pulley. In the continuously variable transmission, the gear ratio is changed generally by changing the hydraulic pressure supplied to the primary pulley, and more specifically, the hydraulic pressure supplied to a primary sheave of the primary pulley.
For example, as the hydraulic pressure supplied to the primary sheave of the primary pulley is increased, the groove width of the primary pulley is narrowed. Consequently, the effective diameter of the primary pulley is increased. In accordance with an increase in the effective diameter of the primary pulley, the groove width of the secondary pulley is increased while the effective diameter thereof is decreased. Consequently, the continuously variable transmission upshifts.
In contrast, as the hydraulic pressure supplied to the primary sheave of the primary pulley is decreased, the groove width of the primary pulley is increased. Consequently, the effective diameter of the primary pulley is decreased. In accordance with a decrease in the effective diameter of the primary pulley, the groove width of the secondary pulley is narrowed while the effective diameter thereof is increased. Consequently, the continuously variable transmission downshifts.
The pressure for holding a metal belt and the like is adjusted by the hydraulic pressure supplied to the secondary sheave of the secondary pulley, for example, as disclosed in Japanese Patent Laying-Open No. 2005-163934.
Generally, the pressure for holding the metal belt is adjusted in accordance with the driving state of the vehicle so as to prevent the metal belt from slipping. In order to implement the pressure suitable in accordance with each of various driving states, a plurality of control modes are used, in each of which a target value of the hydraulic pressure is set. For example, in one control mode, the greater the input torque of the continuously variable transmission is, the more the pressure for holding the metal belt, that is, the hydraulic pressure supplied to the secondary pulley, is raised.
In another control mode, during deceleration in the state where a brake pedal is depressed, the hydraulic pressure supplied to the secondary pulley is increased as compared with the case where the brake pedal is not depressed. Consequently, the metal belt can be prevented from slipping during braking.
In addition, when the target value of the hydraulic pressure changes, the actual hydraulic pressure may undershoot or overshoot the target value. One approach for preventing such undershoot and overshoot is to gradually change the target value of the hydraulic pressure.